This invention relates to radio reception, and more particularly relates to the squelching of a signal received by a radio.
Squelching techniques have been known in the past. Typically, such techniques are based on an envelope detected signal which has already been subjected to automatic gain control. That is, the squelching is based on a normalized envelope detected signal. In addition, frequency division of signal and noise is used in most commercial radios.
New regulations confining aviation radio channels to an 8.33 KHz channel separation have decreased the performance capability of prior known squelch techniques. The 8.33 KHz channel separation makes the frequency separation of signal plus noise power and noise power for use in squelch operation unreliable due to carrier frequency uncertainty and the closeness of adjacent channels. As a result, a reliable method of detecting signal presence prior to baseband AGC and of deriving a measure of signal plus noise power divided by signal power is desired. Such method must not overload the processor executing baseband processing. The present invention addresses these problems and provides a solution.
The preferred embodiment is useful in a radio receiver adapted to receive a radio frequency carrier signal. In such an environment, signal squelching is provided by generating a baseband carrier signal in response to the radio frequency carrier signal. A detected signal is generated in response to the baseband carrier signal and a noise estimate signal is generated by estimating at least the thermal noise in the detected signal. A threshold signal defining a predetermined signal level is generated in response to the noise estimate signal and one or more predetermined signal parameters. A first signal is generated in the event that a signal derived from the detected signal has a first predetermined relationship with respect to the predetermined signal level, and a second signal is generated in the event that the signal derived from the detected signal has a second predetermined relationship with respect to the predetermined signal level. The detected signal is passed in response to the first signal, and the detected signal is squelched in response to the second signal.
By using the foregoing techniques, a squelch algorithm of acceptable computational complexity for a CPU processor can be provided. An estimate of signal quality can be made even with an 8.33 KHz channel spacing. The estimate does not require extensive filtering to separate signal power plus noise and noise power alone.